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908JK3 unexpected port pulse at power up.

Question asked by Graham Pratt on Jul 14, 2010
Latest reply on Jul 14, 2010 by Graham Pratt

Hi all. :smileyhappy:

Got a new project up and going using a 908JK3 as part of an ignition coil driver. Works extremely well, particularly controlling dwell under transient rpm changes at low speeds. Anyhoo... on power up there is an unintended 2 volt 10mS pulse coming out of port B4. (maybe others too but not using them) At power up this causes single spark from the coil. Chip is running on 5 volts and there is a 10k resistor from this port to the base of a PN100 transistor. I put a 1k resistor as a load from port to ground and the pulse is reduced from 2 volts to 200mV and the problem is "solved". :smileyindifferent: I am setting the ports to initial values ~before~ writing to the DDRs to avoid the glitch problem the datasheet mentions. Also I noticed that if I hold the reset line low it still happens so it is not the DDR thing mentioned above. I'm sure this thing is probably well known.